Synthesis and biological evaluation of (+)-neopeltolide analogues: Importance of the oxazole-containing side chain

We describe the synthesis and biological evaluation of (+)-neopeltolide analogues with structural modifications in the oxazole-containing side chain. Evaluation of the antiproliferative activity of newly synthesized analogues against A549 human lung adenocarcinoma cells and PANC-1 human pancreatic carcinoma cells have shown that the C19–C20 and C26–C27 double bonds within the oxazole-containing side chain and the terminal methyl carbamate group are essential for potent activity.     

Positioning of the carboxamide side chain in 11-oxo-11H-indeno[1,2-b]quinolinecarboxamide anticancer agents: effects on cytotoxicity

A series of 11-oxo-11H-indeno[1,2-b]quinolines bearing a carboxamide-linked cationic side chain at various positions on the chromophore was studied to determine structure-activity relationships between cytotoxicity and the position of the side chain. The compounds were prepared by Pfitzinger synthesis from an appropriate isatin and 1-indanone, followed by various oxidative steps, to generate the required carboxylic acids. The 4- and 6-carboxamides (with the side chain on a terminal ring, off the short axis of the chromophore) were effective cytotoxins. The dimeric 4- and 6-linked analogues were considerably more cytotoxic than the parent monomers, but had broadly similar activities. In contrast, analogues with side chains at the 8-position (on a terminal ring but off the long axis of the chromophore) or 10-position (off the short axis of the chromophore but in a central ring) were drastically less effective. The 4,10- and 6,10-biscarboxamides had activities between those of the corresponding parent monocarboxamides. The first of these showed good activity against advanced subcutaneous colon 38 tumours in mice.     

Single-channel studies on linear gramicidins with altered amino acid side chains. Effects of altering the polarity of the side chain at position 1 in gramicidin A.

The modulation of gramicidin A single-channel characteristics by the amino acid side chains was investigated using gramicidin A analogues in which the NH2 terminal valine was chemically replaced by other amino acids. The replacements were chosen such that pairs of analogues would have essentially isosteric side chains of different polarities at position 1 (valine vs. trifluorovaline or hexafluorovaline; norvaline vs. S-methyl-cysteine; and norleucine vs. methionine). Even though the side chains are not in direct contact with the permeating ions, the single-channel conductances for Na+ and Cs+ are markedly affected by the changes in the physico-chemical characteristics of the side chains. The maximum single-channel conductance for Na+ is decreased by as much as 10-fold in channels formed by analogues with polar side chains at position 1 compared with their counterparts with nonpolar side chains, while the Na+ affinity is fairly insensitive to these changes. The relative conductance changes seen with Cs+ were less than those seen with Na+; the ion selectivity of the channels with polar side chains at position 1 was increased. Hybrid channels could form between compounds with a polar side chain at position 1 and either valine gramicidin A or their counterparts with a nonpolar side chain at position 1. The structure of channels formed by the modified gramicidins is thus essentially identical to the structure of channels formed by valine gramicidin A. The polarity of the side chain at position 1 is an important determinant of the permeability characteristics of the gramicidin A channel. We discuss the importance of having structural information when interpreting the functional consequences of site-directed amino acid modifications.     

Structural optimization and evaluation of butenolides as potent antifouling agents: modification of the side chain affects the biological activities of compounds

A recent global ban on the use of organotin compounds as antifouling agents has increased the need for safe and effective antifouling compounds. In this study, a series of new butenolide derivatives with various amine side chains was synthesized and evaluated for their anti-larval settlement activities in the barnacle, Balanus amphitrite. Side chain modification of butenolide resulted in butenolides 3c-3d, which possessed desirable physico-chemical properties and demonstrated highly effective non-toxic anti-larval settlement efficacy. A structure-activity relationship analysis revealed that varying the alkyl side chain had a notable effect on anti-larval settlement activity and that seven to eight carbon alkyl side chains with a tert-butyloxycarbonyl (Boc) substituent on an amine terminal were optimal in terms of bioactivity. Analysis of the physico-chemical profile of butenolide analogues indicated that lipophilicity is a very important physico-chemical parameter contributing to bioactivity.     

Thrombomodulin induction in cultured human endothelial cells by 9-cis-locked retinoic acid analogues

9-cis-Retinoic acid (RA) analogues devised to lock the 9-cis double bond by ring formation were synthesized using two stereoselective carbon-carbon bond formation reactions as key steps. The palladium-mediated Suzuki reaction was adopted to construct a 7E-double bond (RA numbering) and the Horner-Emmons olefination was employed for stereoselective 11E-double bond (RA numbering) formation. The synthesized 9-cis-RA analogues that are locked by five-membered ring systems (cyclopentene, dihydrofuran, and dihydrothiophene) were shown to have comparable thrombomodulin induction activities to that of 9-cis RA. Conformational analysis of these compounds showed their similarity to 9-cis RA in the spatial orientation of the side chain and the terminal carboxy group.     

Structural optimization and evaluation of butenolides as potent antifouling agents: modification of the side chain affects the biological activities of compounds

A recent global ban on the use of organotin compounds as antifouling agents has increased the need for safe and effective antifouling compounds. In this study, a series of new butenolide derivatives with various amine side chains was synthesized and evaluated for their anti-larval settlement activities in the barnacle, Balanus amphitrite. Side chain modification of butenolide resulted in butenolides 3c-3d, which possessed desirable physico-chemical properties and demonstrated highly effective non-toxic anti-larval settlement efficacy. A structure-activity relationship analysis revealed that varying the alkyl side chain had a notable effect on anti-larval settlement activity and that seven to eight carbon alkyl side chains with a tert-butyloxycarbonyl (Boc) substituent on an amine terminal were optimal in terms of bioactivity. Analysis of the physico-chemical profile of butenolide analogues indicated that lipophilicity is a very important physico-chemical parameter contributing to bioactivity.     

Synthesis and cytotoxic activity of acetogenin analogues

A set of 16 new simplified analogues of acetogenins has been designed based on: (i) the replacement of the bis THF moiety of these natural products by an ethylene glycol bis ether unit; (ii) the introduction of different lipophilic side chains (alkyl, aryl, dialkylamino, O-cholesteryl); (iii) the presence of the same terminal isolactone. In vitro cytotoxic activity against L1210 leukemia is reported.     

Natural cannabinoids: templates for drug discovery

Recent studies have elucidated the biosynthetic pathway of cannabinoids and have highlighted the preference for a C-3 n-pentyl side chain in the most prominently represented cannabinoids from Cannabis sativa and their medicinally important decarboxylation products. The corresponding C-3 n-propyl side chain containing cannabinoids are also found, although in lesser quantities. Structure-activity relationship (SAR) studies performed on Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the key psychoactive ingredient of Cannabis, and its synthetic analogues have identified the C-3 side chain as the key pharmacophore for ligand affinity and selectivity for the known cannabinoid receptors and for pharmacological potency. Interestingly, the terminal n-pentyl saturated hydrocarbon side chain of endocannabinoids also plays a corresponding crucial role in conferring similar properties. This review briefly summarizes the biosynthesis of cannabinoids and endocannabinoids and focuses on their side chain SAR.     

Long-chain formoterol analogues: an investigation into the effect of increasing amino-substituent chain length on the beta2-adrenoceptor activity

The synthesis of a series of long-chain formoterol analogues in which the terminal ether residue of the beta-phenethyl-amino-substituent has been extended beyond the methyl ether residue present in the parent compound are described. Evaluation of these analogues as beta(2)-adrenoceptor agonists was used to provide an insight into the factors controlling the magnitude and duration of receptor activation.     

Synthesis and activity of analogues of the isoleucyl tRNA synthetase inhibitor SB-203207

Twenty two analogues of SB-203207 have been prepared by total synthesis, and evaluated as inhibitors of a range of tRNA synthetases. Changes to the bicyclic core, removing either the terminal amino substituent or the sulfonyl group from the side chain, and altering either the carbon skeleton or stereochemistry of the isoleucine residue, decreases the potency of inhibition of isoleucyl tRNA synthetase. Substituting the isoleucine residue with other amino acids produces inhibitors of the corresponding synthetases. In particular, a methionine derivative is 50-100 times more potent against methionyl tRNA synthetase than against any of the corresponding isoleucyl, leucyl, valyl, alanyl and prolyl synthetases.     

STRUCTURE ACTIVITY RELATIONS FOR THE INHIBITION OF 5-HT UPTAKE INTO RAT HYPOTHALAMIC HOMOGENATES BY SEROTONIN AND TRYPTAMINE ANALOGUES

Abstract— Various 5-HT and tryptamine analogues have been examined as inhibitors of [³H]5-HT uptake into rat hypothalamic homogenates. Acetylation of the terminal amino group or methylation of the hydroxyl group of 5-HT resulted in compounds having a reduced affinity for the serotonin uptake site. This also occurred when the hydroxyl group of 5-HT was substituted in other positions on the benzene ring. Substitution of the tryptamine side chain in the a-position by methyl or ethyl groups, but not by a carboxyl function, enhanced the affinity for the 5-HT uptake site. Increasing the tryptamine side chain by one carbon atom also resulted in a more potent compound. Several of the compounds tested are known to be either hallucinogens or antidepressants.     

Thermodynamic stability of HLA-B*2705. Peptide complexes. Effect of peptide and major histocompatibility complex protein mutations

Designing synthetic vaccines from class I major histocompatibility complex (MHC)-binding antigenic peptides requires not only knowledge of the binding affinity of the designed peptide but also predicting the stability of the formed MHC-peptide complex. In order to better investigate structure-stability relationships, we have determined by circular dichroism spectroscopy the thermal stability of a class I MHC protein, HLA-B*2705, in complex with a set of 39 singly substituted peptide analogues. The influence of two anchoring side chains (P3 and P9) was studied by peptide mutation and appropriate site-directed mutagenesis of the HLA-B*2705 binding groove. The side chain at P9 is clearly the one that contributes the most to the thermal stability of the MHC-peptide complexes, as destabilization up to 25 degrees C are obtained after P9 mutation. Interestingly, structure-stability relationships do not fully mirror structure-binding relationships. As important as the C-terminal side chain are the terminal ammonium and carboxylate groups. Removal of a single H-bond between HLA-B27 and the terminal peptide moieties results in thermal destabilization up to 10 degrees C. Depending on the bound peptide and the location of the deleted H-bond, the decrease in the thermal stability of the corresponding complex is quantitatively different. The present study suggests that any peptidic amino acid at positions 3 and 9 promotes refolding of the B27-peptide complex. Once the complex is formed, the C-terminal side chain seems to play an important role for maintaining a stable complex.     

Dehydroquinate synthase: the use of substrate analogues to probe the early steps of the catalyzed reaction

The early steps of the proposed mechanistic pathway for dehydroquinate synthase have been probed with a series of substrate analogues. These analogues, 3-9, are structurally prohibited from undergoing the beta-elimination of inorganic phosphate that represents the committed step in the conversion of the substrate 3-deoxy-D-arabino-heptulosonate 7-phosphate (1) to dehydroquinate (2). In agreement with previous observations, the analogues that possess shortened side chains (3,5, and 6) bind more tightly to the enzyme than those (4 and 7-9) that are more nearly isosteric with the substrate. Two hitherto unrecognized factors that influence binding have been identified: (i) carbacylic analogues bind 25-100 times more tightly than the corresponding oxacyclic materials (indeed, the carbacyclic phosphonate 5 has a Ki value of 8 x 10(-10)M) and (ii) the side chain appears to be bound in a gauche conformation similar to the most stable conformation of the cis-vinylhomophosphonate 8. These trends in binding can be rationalized by considering the behavior of the analogues in the first two chemical steps of the mechanism: NAD+-mediated oxidation at C-5 and enolization at C-6 (the first part of the E1cB elimination of inorganic phosphate). Direct spectrophotometric determination of the equilibrium level of enzyme-bound NADH indicates that the carbacyclic analogues are more readily oxidized than the oxacyclic compounds, and this predictable difference in redox behavior is reflected in the observed differences in binding. The gauche conformation of the C-7 side chain appears to be required for proton abstraction from C-6, since only those analogues that can adopt this conformation undergo enzyme-catalyzed exchange of the C-6 proton with the solvent. This conformation positions one of the peripheral oxygens of the phosphate (or phosphonate) group close to the C-6 proton. Taken together with other data, these results suggest that the enzyme exploits this substrate base in the enolization, which occurs through an intramolecular proton transfer. The loss of Pi then completes the beta-elimination.     

Interactions between prostaglandin analogues and a receptor in bovine Corpora lutea. Correlation of dissociation constants with luteolytic potencies in hamsters.

The dissociation constants for the interactions between some prostaglandin analogues and a prostaglandin F2 receptor in bovine corpora lutea were determined. These values were compared to the antifertility potencies of these compounds in hamsters and the rates of metabolism by 15-hydro-syprostaglandin dehydrogenase. The most active analogues with regard to both affinity for the receptor and luteolytic potency were 17-phenyl-18, 19, 20-trinorprostaglandin F2alpha and 15-methylprostaglandin F2alpha. The alkyl side chain of prostaglandins could be modified considerably without altering the affinity for the receptor. In this way metabolism by 15-hydroxyprostaglandin dehydrogenase could be blocked. Some of these compounds -ad greatly increased luteolytic effects. Substitution of a phenyl group for the 3 terminal carbon units of the alkyl side chain of prostaglandins increased both the affinity for the receptor and the luteolytic activity in vivo. 7-oxa-13-prostynoic acid, an antagonist of the luteolytic effect of prostaglandin F2alpha in vivo was a weak competitive inhibitor of the interation between prostaglandin F2alpha and the receptor.     

Identification of ACE pharmacophore in the phosphonopeptide metabolite K-26

The naturally occurring phosphonotripeptide K-26 is a potent angiotensin converting enzyme (ACE) inhibitor containing an alpha-amino phosphonic acid analogue of tyrosine. Previous studies have demonstrated that canonical peptide analogues of K-26 are micromolar inhibitors of ACE. To ascertain the structure-activity relationships in this class of ACE inhibitory natural products, K-26 and eight analogues were chemically synthesized and evaluated. Phosphonyl substitution was found to be the critical determinant of activity, resulting in a 1500-fold increase in ACE inhibition versus carboxyl analogues. Secondarily, the absolute configuration of the terminal alpha-amino phosphonate and N-acetylation were found to significantly modulate ACE inhibitory activity.     

The effect of β‐methylation on the conformation of α, β‐dehydrophenylalanine: a DFT study

Dehydroamino acids are non‐coded amino acids that offer unique conformational properties. Dehydrophenylalanine (ΔPhe) is most commonly used to modify bioactive peptides to constrain the topography of the phenyl ring in the side chain, which commonly serves as a pharmacophore. The Ramachandran maps (in the gas phase and in CHCl₃ mimicking environments) of ΔPhe analogues with methyl groups at the β position of the side chain as well as at the C‐terminal amide were calculated using the B3LYP/6‐31 + G** method. Unexpectedly, β‐methylation alone results in an increase of conformational freedom of the affected ΔPhe residue. However, further modification by introducing an additional methyl group at C‐terminal methyl amide results in a steric crowding that fixes the torsion angle ψ of all conformers to the value 123°, regardless of the Z or E position of the phenyl ring. The number of conformers is reduced and the accessible conformational space of the residues is very limited. In particular, (Z)‐Δ(βMe)Phe with the tertiary C‐terminal amide can be classified as the amino acid derivative that has a single conformational state as it seems to adopt only the β conformation. Copyright © 2009 European Peptide Society and John Wiley & Sons, Ltd.     

Antiproliferative activities of a library of hybrids between indanones and HDAC inhibitor SAHA and MS-275 analogues

New compounds derived from inhibitors of histone deacetylases (HDACs) have been synthesized and their antiproliferative activities towards non small lung cancer cell line H661 evaluated. Their design is based on hybrids between indanones to limit conformational mobility and other known HDAC inhibitors (SAHA, MS-275). The synthesis of these new derivatives was achieved by alkylation of appropriate indanones to introduce the side chain bearing a terminal ester group, the latter being a precursor of hydroxamic acid and aminobenzamide derivatives. These new analogues were found to be moderately active to inhibit H661 cell proliferation.     

Toxicity of nucleoside analogues used to treat AIDS and the selectivity of the mitochondrial DNA polymerase

Incorporation of nucleoside analogues by the mitochondrial DNA polymerase has been implicated as the primary cause underlying many of the toxic side effects of these drugs in HIV therapy. Recent success in reconstituting recombinant human enzyme has afforded a detailed mechanistic analysis of the reactions governing nucleotide selectivity of the polymerase and the proofreading exonuclease. The toxic side effects of nucleoside analogues are correlated with the kinetics of incorporation by the mitochondrial DNA polymerase, varying over 6 orders of magnitude in the sequence zalcitabine (ddC) > didanosine (ddI metabolized to ddA) > stavudine (d4T) > lamivudine (3TC) > tenofovir (PMPA) > zidovudine (AZT) > abacavir (metabolized to carbovir, CBV). In this review, we summarize our current efforts to examine the mechanistic basis for nucleotide selectivity by the mitochondrial DNA polymerase and its role in mitochondrial toxicity of nucleoside analogues used to treat AIDS and other viral infections. We will also discuss the promise and underlying challenges for the development of new analogues with lower toxicity.     

The importance of the phospholipid bilayer and the length of the cholesterol molecule in membrane structure.

The properties of mixtures of phosphatidylcholine and analogues of cholesterol bearing side chains of varying lengths were examined by a variety of methods. The incorporation of the analogues into sonicated liposomes and their effect on the rate of osmotic shrinking of multilamellar liposomes were determined. The ordering of a steroid spin label was studied in an oriented multibilayer system and the effect of the analogues on the phase transition of dipalmitoyl phosphatidylcholine monitored using the spin label TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl). Mixtures of analogues and phospholipid were also studied in monolayers. In all the bilayer systems studied cholesterol caused the greatest 'rigidifying' effect, the analogues with shorter or longer side chains being less effective. However, in the monolayer experiments the length of the sterol molecule was found to be much less critical. It is suggested that cholesterol is anchored in position in a phospholipid bilayer by virtue of the molecule being the precise length required to maximise interactions between neighbouring molecules without disturbing the bilayer structure.     

Insulin analogues - state of the art

The use of insulin analogues is rapidly expanding. At present, there are two short-acting analogues available for practical use, insulin aspart and insulin lispro, and one long-acting analogue, insulin glargine. Another long-acting analogue, insulin detemir, is still under development. The time action profile of short-acting analogues is both much more rapid and shorter than that of human insulin; the prominent feature of the long-acting analogues is their peakfree and fairly constant action. Insulin analogues offer alternative options for the whole spectrum of insulin therapy in type 1 and type 2 diabetes patients. The perception by many patients is strikingly positive, in particular regarding the overall quality of life. In objective efficacy terms, however, the potential to improve the degree of metabolic control appears to be only minor, yet demonstrable, provided the analogues are used according to their specific time action profile. This ensured, analogues are instrumental in minimizing the side effects of insulin therapy, i.e. the risk of (nocturnal) hypoglycaemia or problems with body weight control. Although there are no indications of safety concerns with insulin analogues, the availability of long-term outcome data based upon observations in human patients would be very valuable.